First of, what epics do you have leveled? From there you should be able to get some tips on the direction to head. Other than that, I would love to suggest that you check out the Fatal Frontier Wiki. On the site their is a tier list that will tell you roughly which guys are good to shoot for and which ones to avoid. I'll link you to the lists, and feel free to explore the wiki from there.

As always, if you have any more questions, just throw something out, and if you want to we have a forum section just for introductions called the cardinal enclave. If you make a little thread introducing yourself there people are usually a little better about responding

The problem is, these mercs are hard to sell, and aren't worth a lot. When you consider a v2 Lethal costs at least ~300 AC, you're still a long way from that. These prices may not be completely correct, but considering nobody would even buy a Power Lancer, you're not even close to the value of a Lethal. If they're not PE ( Perfect Evolution = 8in1 ), this may even decrease the value. You should aim for mercs like Incinerate, Cannonback, etc. If you can pick these up cheap and sell high, then you'll get Lethals in no-time.